Transmission Modes

Every mode on this page is the same basic idea: your radio takes an audio signal — your voice, a key click, or tones from a computer — and uses it to bend a radio-frequency carrier in a predictable way. The receiver bends it back. The "mode" is simply the recipe for the bending, and it decides how much power you need, how much spectrum you use, and what you can get through on a weak signal.

Basic modes

USB — upper sideband

Keep the sideband above the carrier and discard the carrier and its mirror. The result is a voice signal about 3 kHz wide that puts nearly all of its power into your voice. USB is the default for HF voice on 20 meters and up — most of the casual chatting on the bands happens there — and it's also how most digital modes ride the air, since the computer's tones modulate a USB signal.

LSB — lower sideband

The same idea, keeping the sideband below the carrier. By convention, HF voice below 10 MHz uses LSB — that's how you'll find phone on 80 and 160 meters. Above 10 MHz, switch to USB. The one notable exception in the U.S. is the 60 meter band, where the rules specifically require USB.

AM — amplitude modulation

Double-sideband AM, the original "AM radio" scheme: the carrier plus both sidebands, so a voice signal is about 9 kHz wide and a large share of your power sits in the carrier, which carries no information. It's the simplest mode to build and receive, and you still hear it on some HF bands and in parts of Europe — but for distance work, SSB wins on every count.

FM — frequency modulation

Narrowband FM: your audio varies the carrier's frequency instead of its amplitude. That makes it highly resistant to noise and fading at the cost of a 12.5–25 kHz channel. FM is how everyone talks on 2 and 70 centimeters — handhelds, simplex, and repeaters — and HF has small FM subbands on 80, 40, and 20 meters. Repeater access is usually gated by a sub-audible CTCSS tone, which your radio transmits alongside your voice.

CW — Morse code

The oldest mode, and still the most efficient. You key a pure carrier on and off with a straight key or paddle, and the signal is only about 100 Hz wide — far narrower than any voice mode — so a weak signal at a few watts is still copyable. If you want to work the world on QRP, this is the mode to learn.

Digital modes

RTTY — radioteletype

The original keyboard-to-keyboard radio mode, evolved from teleprinter circuits of the 1920s and 40s. Each bit is one of two audio tones (mark and space) — the classic "beedle-beedle-beedle" sound — and the amateur standard is 45.45 baud with a 170 Hz shift, using the old five-bit Baudot code. A typical signal needs about 250 Hz of receiver bandwidth, it's forgiving on HF propagation, and because its power is constant it doesn't need a linear amplifier. It remains a favorite for weak-signal DX; software like fldigi or MMTTY does the work the old teleprinters used to.

PSK31

Phase-shift keying at 31.25 baud: instead of two tones, the phase of a single tone jumps to encode bits, so the whole signal fits in about 300 Hz — narrower than RTTY at a similar typing speed. Developed by G3RUH for amateurs, it supports break-in, so two stations can exchange text in real time without taking turns in long bursts. It's a gentle first step from RTTY into the modern digital world.

FT8 / FT4

The weak-signal workhorses, designed by Joe Hall (G4WJS). FT8 structures the contact into 15-second transmissions: each station sends a short 13-character message encoding callsign, grid square, and signal report, and the software can find it more than 28 dB below the noise floor. FT4 is the same idea with a 7.5-second cycle, for faster exchanges. If you've ever watched someone work a station they apparently couldn't hear, it was FT8.

WSPR

A propagation-reporting mode, also from K1JT: a short message repeated on a 110-second cycle, decodable about 24 dB below the receiver threshold. Stations around the world listen at the same moment and log where they heard you — a free, global map of how the bands are propagating right now.

APRS and packet radio

Amateur packet radio (AX.25) sends data as 1200-baud tones over FM. APRS builds on it: your GPS position and status ride in small packets on the 2 meter band (144.390 MHz in the U.S.) and appear on maps like aprs.fi. A station connected to the internet is an "igate," relaying local packets to the global network.

SSTV — slow-scan TV

A still image encoded as audio tones and sent in a minute or two; the receiving software rebuilds the picture. One of the most rewarding first digital contacts — and we have a build guide in the projects section.

ATV — fast-scan TV

Live, real-time video over roughly 6 MHz of bandwidth. In the U.S. it's mostly a 70 centimeter activity, in analog and digital forms.

Olivia

An MFSK mode with strong forward error correction, built to get copies through band conditions where nothing else will — a staple of the big DXpeditions. You'll often hear it called "polytone."

PACTOR

A packet mode with forward error correction, developed by German amateurs. Run through Winlink, it handles high-speed file transfer and email over HF — the practical choice when you need to move real data, not just a short message.

Hellschreiber (OLDMODE)

A 1930s FAX-style mode: 15 baud, 100 Hz shift, printed as dots on paper. You'll still occasionally hear it on HF under the name OLDMODE — a throwback the hobby keeps alive.

Crazy propagation

The ionosphere, the moon, and the weather conspire to send signals places they have no business going. These are the mechanisms behind the strangest contacts on the air.

Meteor scatter — meteor shower bounce

Meteoids burning up in the upper atmosphere leave brief trails of ionized gas that reflect VHF and UHF signals. Tune 2 meters or 6 meters during a meteor shower and you can catch contacts that flicker in and out as trails form and fade. On HF, the steady drizzle of sporadic small meteors produces "meteor burst" communication — a steady scatter of weak, bouncy signals that's its own thing entirely.

Moonbounce — EME

Your signal travels 384,000 km to the moon, bounces, and comes back — another 384,000 km to the other station. That round trip costs over 100 dB, so EME is a game of big antennas, linear amplifiers, and slow digital modes (or patient CW). Work a station with the moon as the reflector and you'll understand why EME operators are the hobby's long-distance obsessives. It's done mostly on 2 and 70 centimeters, whenever the moon is up for both stations.

Sporadic E

Thin, patchy layers of intense ionization appear out of nowhere in the E layer, usually in summer, and turn 2 and 6 meters into long-distance bands for hours at a time. When "Es" opens, you can work the other side of the continent on modest power. It's the most rewarding day on the VHF calendar.

Aurora

During geomagnetic storms, the aurora's ionization scatters VHF signals with a characteristic smeared, warbly sound — especially on CW — while chewing up the lower HF bands in absorption. If you copy a station on 2 meters with a voice like it's being dragged across gravel, you're on aurora.

Tropospheric ducting and trapped energy

Temperature inversions in the troposphere bend VHF and UHF signals along the horizon, carrying 2 m, 6 m, 70 cm, and even 23 cm signals hundreds to thousands of kilometers. In summer, a related effect called trapped energy keeps 6 and 2 meters alive for hours at a stretch — the VHF version of a good HF day.

NVIS

Near-vertical incidence skywave: point your radiation straight up and let the signal bounce back down all around you, filling in the "skip zone" where normal skywave and ground wave don't reach. 160 meter NVIS in winter is the classic setup for covering your whole region at once.

Whistler mode

Lightning on the other side of the planet can launch electromagnetic waves that ride Earth's magnetic field lines to you, arriving as a descending whistle a second or two later. Hams have listened to whistlers for a century, and the military even experimented with them as a communication channel in the 1960s. Not a mode you'll work — but the strangest thing on this page.

Quick reference

Bandwidth figures are approximate — a rough guide for tuning, not a specification.

Resources

• Wikipedia — Single-sideband modulation
• Wikipedia — Amplitude modulation
• Wikipedia — Frequency modulation
• Wikipedia — Radioteletype (RTTY)
• Wikipedia — PSK31
• Wikipedia — WSJT (FT8, WSPR, and friends)
• Wikipedia — Automatic Packet Reporting System
• Fldigi — multi-mode digital software
• ARRL — the Handbook and frequency guides